1. Field of the Invention
The present invention relates to an organic electro-luminescence device, and more particularly, to an organic electro-luminescence device for improving picture quality and providing high gradation, and a method of driving the same.
2. Description of the Related Art
To replace heavy and bulky cathode ray tubes (CRTs), various kinds of flat panel displays have been recently developed.
Examples of the flat panel displays are a liquid crystal display (LCD), a field emission display (FED), a plasma display panel (PDP), an electro-luminescence display (ELD), etc. Many attempts have been made to provide an enhanced display quality and large screen of the flat panel displays.
Among the flat panel displays, the organic electro-luminescence device is a self-luminous display device that emits light by itself. The organic electro-luminescence device displays a predetermined image by exciting a phosphor material using carriers, such as electrons and holes. Accordingly, the organic electro-luminescence device can be driven at a low voltage and have a high response speed.
FIG. 1 is a circuit diagram illustrating a pixel structure of a related art organic electro-luminescence device.
The pixel structure of the organic electro-luminescence device driven by a voltage addressing is illustrated in FIG. 1.
Referring to FIG. 1, a driving transistor M1 is connected to an organic light-emitting diode (OLED) and supplies a driving current IOLED used for emission of light. An amount of the driving current IOLED of the driving transistor M1 is controlled by a data voltage applied through a switching transistor M2. At this point, a capacitor C1 for maintaining the applied voltage during a predetermined period is connected between source and gate of the driving transistor M1. Also, the switching transistor M2 has a gate connected to a gate line Sn, a source connected to a data line Dm, and a drain connected to the gate of the driving transistor M1.
Upon the operation of the organic electro-luminescence device with the above pixel structure, the switching transistor M2 is turned on by a select signal applied to the gate thereof, and a data voltage from the data line Dm is applied to the gate of the driving transistor M1. Then, the driving current IOLED corresponding to a voltage VGS charged between the gate and the source of the driving transistor M1 flows through the driving transistor M1. The OLED emits light in response to the driving current IOLED. The driving current flowing through the OLED is expressed as
      (          I      OLED        )    =            β      2        ⁢                  (                              V            DD                    -                      V            DATA                    -                                                V              TH                                                  )            2      
where IOLED, Vth, VDATA, and β represent the driving current flowing through the OLED, a threshold voltage of the transistor M1, a data voltage, and a constant value, respectively.
According to the pixel structure of FIG. 1, the driving current corresponding to the data voltage is applied to the OLED, and then the OLED emits light in response to the applied driving current. The applied data voltage has a multi-step value within a predetermined range to express gradation.
In the related art pixel structure, the deviation of the threshold voltage Vth and the electron mobility in a thin film transistor is caused by non-uniformity of the manufacturing processes. Therefore, the luminance deviation occurs in each pixel, resulting in non-uniformity of the picture quality. Consequently, the picture quality is degraded. For example, when the thin film transistor of the pixel is driven at 3 V, 8-bit (256) gradation can be expressed by applying a voltage to the gate of the thin film transistor at intervals of 12 mV (3V/256). However, if the deviation of the threshold voltage in the thin film transistor is 100 mV due to non-uniformity of the manufacturing processes, the levels of the gradation for the thin film transistors with deviation of the threshold voltage would be reduced.